The present invention relates to a process for the preparation of photographic color copies from photographic copy masters in a photographic color copy apparatus.
Photographic color copies are, for example, prepared as paper images from photographic copy masters such as negative films of slides. However, it is normally not possible to work with standardized, uniform quantities of copying light in preparing these color copies. The copy masters are therefore analyzed in photographic copy apparatuses to determine color and density corrections. The corrections are determined from the analytical data, from which the necessary copy light quantities, in particular for the three primary colors of blue, green and red, may in turn be determined. In this manner, exposure errors in the production of the copy master may be compensated in the color copy. In addition to genuine false exposures, expressed by extremely under or overexposed copy masters, color tints due to unfavorable illumination (for example neon light) or the use of unsuitable or degraded film material, may also be compensated. The exposure control processes employed for this purpose are essentially based on two steps:
(a) The copy master is analyzed spectrally. The purpose of the analysis is on the one hand to distinguish the areas of the copy master essential for the image from those of lesser importance and on the other, to detect color tints due to the carrier material, unfavorable illumination, erroneous exposures, etc.
(b) In keeping with the analytical data, copy light quantities for the three primary colors blue, green and red are determined in order to compensate for color tints and to reproduce at least the areas important for the image of the copy master with the correct color and density.
An exposure control process of this type is described for example in EP-A-312 499. In this known process, the fundamental progression of which is shown in FIG. 2, the copy master is scanned in intervals, preferably point by point, and the measuring light of each scanning interval spectrally analyzed. From the spectral values f, color extract values a of the copy master adapted to the spectral sensitivity variation of the copy material are formed for the primary colors blue, green and red by electronic or computer filtering. By comparing these color extract values with reference densities d* (e.g., of a reference copy master), density deviations c* in the three primary colors blue, green and red are obtained. It should be noted that the reference densities d* are also determined on the basis of the spectral sensitivities of the copy material used.
The density deviations c* represent the input data for the color and density correction procedure D* proper. To be able to apply standardized correction routines, it is recommended to reduce the dependence of the density deviations on the copy material used prior to the procedure. For this, the density deviations c* are combined with a so-called paper matrix P. In the color and density correction procedure D*, correction values .sup.2 c are produced from the density deviations c* possibly subjected first to a transformation P. These correction values in an ideal case correspond to the deviations of the color densities of the copy master to be copied from those of a copy master of the same scene produced under optimum conditions. From the correction values .sup.2 c, the necessary copying light quantities and exposure times are determined for the primary colors of blue, green and red.
Subsequently, the copying light quantities and exposure times are adapted to the spectral sensitivities of the copy material used. For this, the output data of the color and density correction procedure D*, (i.e., the correction values .sup.2 c in the three primary colors blue, green and red), are combined with an exposure matrix B*, which cancels out the effect of the matrix P. Preferably, the exposure matrix B* corresponds to the inverse matrix of the paper matrix P. In actual practice, it is also used frequently to correct erroneous effects of the overall system.
While this known exposure control process compensates incorrect exposures and color tints in the color copy, it has a number of disadvantages. The evaluation of the spectral values of each scanning interval relative to the spectral sensitivity variation of the copy materials requires a special evaluating matrix S, a paper matrix P, and an exposure matrix B* for each of the copy materials used. Different color extract values a are thus obtained for the same scanning interval of the copy master, depending on the type of copy material used. The same is obviously true for the reference densities d*. For every copy material used new reference densities adapted to the prevailing spectral sensitivities must therefore be determined.
The density deviations resulting from the comparison of the aforementioned color extract values and the reference densities are also dependent on the sensitivity variation of the copy material used. In the extreme case this signifies that for each copy material used, a specific color and density correction procedure adapted to the material must be employed. For this reason, numerous attempts are made to reduce the copy material dependence of the density deviations, which represent the input data of the color and density correction routines, by combining them with a 3.times.3 paper matrix. While it is possible in this manner to work with only one standardized routine, a specific paper matrix P and a corresponding exposure matrix B* must be determined for each copy material used. Even in this fashion the input data and the standardized color and density correction routine cannot be optimally reconciled for every copy material, which in turn may lead to the determination of less than optimal or even incorrect copy light quantities.
There is therefore an urgent demand for the alteration and improvement of a process for the preparation of photographic color copies from a photographic copy master, whereby the aforementioned disadvantages are eliminated. The process should be simple and applicable to any customary type of copy material.